Field of the Invention
Various methods exist for measuring strain, curvature or displacement of materials or structural members. One well-known method is to measure stress on or in the members using resistive strain gauges arranged on the surface in patterns such that the bending can be inferred from a knowledge of the modulus of elasticity of the member. Under some conditions it is advantageous to measure stress or deformation using optical fibers. Fibers are ideal for many applications because they can be relatively inert to environmental degradation, are light in weight, are not affected by electromagnetic interference, carry no electrical current, and can be very small and flexible, thus having little or no effect on the structure in which they are embedded. It is possible to either cement fibers to surfaces or to embed them inside, such as in fiber/epoxy composites, concretes, or plastics.
Many types of optical fiber sensors have been developed for the measurement of stress and position. Most employ interference techniques to measure changes in length or bend radius of the fiber. Most of these techniques rely on detecting standing waves set up in the fiber by reflecting part of the light back from its distal end. These techniques are very sensitive (comparable to strain gauges) but require complex and expensive measurement techniques such as interferometry or optical time domain reflectometry (OTDR) for their execution. Measurements are very sensitive to changes in temperature, requiring elaborate compensation techniques. Another limitation of many of the interference techniques is insensitivity to direction because the measurement is made by counting the number of interference peaks due to distortion of a fiber. Thus, for example, shortening of the fiber is indistinguishable from elongation, or bending up is the same as bending down; unless the fibers are arranged in appropriate curves or other special geometric arrangements.
Equipment for performing interference measurements tends to be bulky and expensive, requiring frequent adjustment. It must be capable of distinguishing peaks at spacings of the order of 0.5 to 1 micron or less. This has limited most fiber optic stress measurements to tests which can be performed under carefully controlled laboratory conditions.
Non-interference techniques can be used to measure bending in fiber optics. It is well known that light leaks out of the core of an optical fiber if it impinges on the cladding at a sufficiently large angle with respect to the long axis of the fiber. For every fiber, there is a critical angle dependent on the indices of refraction of core and cladding, beyond which light will escape. If the fiber is bent, some of the light in the core will exceed this angle and escape. This effect has been used to build "microbending" sensors, which simply measure the percentage of transmission of light down a fiber. These suffer from relative insensitivity (little light is lost) at small angles. Usually a microbend sensor consists of a fiber placed in a corrugated fixture such that a force applied to the fixture will create many sharp bends in the fiber. Microbend sensors are used to measure pressures, forces, and displacement. These sensors also do not measure the direction of the force unless pre-tension is applied.
Other fiber optic sensors have been constructed in which the cladding is removed from the core, or the cladding and some of the core are etched away. These sensors may be more sensitive to bending than untreated fibers, but, like other bending sensors mentioned above, give no information about the direction of a bend unless they are bent at rest. They are thus unsuitable for incorporating in a simple manner in composite structures containing many parallel fibers with sensory and structural properties.
Other fiber optic sensors have been constructed which use thin films in place of the cladding, to give location information based on the wavelength of the filter produced by the thin film. This technique shows no improvement in sensitivity over other fiber optic sensing techniques, so interference techniques must be used to obtain useful outputs.
Many sensors are based on measurements of strain, which is basically an elongation of material. Although it is possible to use multiple strain gauges to infer curvature from strain, it is more desirable in many circumstances to measure curvature directly. Often, it is desirable to mount a sensor near the neutral axis of a beam, where there is no strain associated with curvature of the beam. Often, curvature is the parameter of direct interest, such as when measuring deviation from straightness in a pipe or rod. It is also frequently desirable to measure displacement between two structures, which can be inferred from the curvature of a flexible beam or fiber connecting them. Just as strain gauges can be used to infer curvature in some circumstances, curvature sensors can be used to infer strain.
Strain gauges have found wide application in a huge variety of measurement tasks; curvature sensors potentially have just as many applications. The following examples cover only some of the potential applications for fiber optic curvature and displacement sensors: measuring flutter and deflection in aircraft wings and aerospace truss structures; measuring deflections on cranes and lifting devices; measuring movement of bridges, dams, and buildings due to earthquakes, settlement, or other degradation; measuring sag and deflections of pipes, rods, cables, and beams; measuring the effects of frost heave on roadways and runways; sensing traffic movements and soil settlement; measuring wind forces on masts and towers; sensing parameters of sports equipment including skis, poles, shoes, fishing equipment, swords, bats, clubs, balls and clothing; measuring deflections on marine equipment including masts, spars, cables, hull plates, struts, and booms; measuring curvatures of vanes, wires, poles and other flexible structures or probes to infer fluid or slurry flow, speed, and direction of movement; measuring vibration and sound levels by means of flexing beams, fibers, or diaphragms; measuring pressures by sensing the curvature of diaphragms or tanks; measuring acceleration in general; measuring deceleration and associated forces for the deployment of airbags; measuring the deflection of support structures to infer applied weight, forces, torques, and deflections; forming multi-degree-of-freedom force and torque sensors; forming input devices for computers including joysticks, keyboards, and levers; measuring joint angles and deflections on robots, automatically guided vehicles, automobiles, trucks, tanks, earth moving equipment, loaders, cranes, ships, airplanes, helicopters, and spacecraft; measuring the deflections of tire treads and other rubber or elastomeric moving parts; measuring door and wheel positions; measuring pedal, vane, rudder, lift surface, and valve positions; measuring shaft and knob angles, rotations, and positions; measuring liquid levels by deflection of floats or bladders; measuring alignment of automotive, marine, or industrial equipment; measuring positions and motion of reclining seats, chairs, beds, and medical fixtures; instrumenting medical tools; instrumenting prosthetic devices; measuring deflections in the presence of high magnetic fields; measuring magnetic and electric fields by virtue of forces or motion generated in magnetic or electric media attached to a curvature sensor; measuring concentration or presence of liquids, gases, and vapours by virtue of dimensional changes induced in a substrate to which the curvature sensor is attached; measuring temperatures by virtue of dimensional changes induced in a substrate to which the curvature sensor is attached; measuring positions and angles of parts of animal, including human, bodies; and many others.
Generally, fiber optic sensors that must be exposed to harsh environments or that must be embedded, should be intrinsic sensors, that is, sensors that do not rely on light leaving and then reentering the fiber. Thus, sensors that involve light exiting a fiber, reflecting off a surface, and re-entering are not desirable for many purposes because the surfaces may become contaminated, thus changing the light intensity.
A wide variety of intrinsic fiber optic sensors has been described, most of them based on interference techniques. Interference-based sensors, which rely on mechanical changes to the fiber dimensions producing changes in light interference patterns within the fiber, are very sensitive to strain but also to temperature and involve complex and expensive electronic circuitry. Another drawback is that usually lasers or laser diodes must be used as light sources with these sensors, thereby limiting their durability and longevity, and increasing their cost.
Optical fibers transmit light by virtue of total internal reflection. The light is contained in a core of transparent material. Generally, this core is covered with a cladding layer that has a lower index of refraction than the core. Because the index of the cladding is lower than that of the core, rays within a certain range of angles of incidence with the core/cladding boundary will be refracted back into the core upon striking the cladding. If the fiber is bent in a curve, small amounts of light are lost due to changes in the angle of incidence at the curved core/cladding boundary. If the curvature becomes substantial, significant amounts of light may be lost. Fibers with a discrete core/cladding boundary are called step index fibers. Other fibers called graded index fibers do not have a distinct boundary between core and cladding, but exhibit a continuous decrease in index of refraction toward the outer circumference of the fiber. For simplicity, this description will use terminology consistent with step index fibers, but graded index fibers may be similarly treated, as may other light guides including guides of non-circular cross section including, for example, a D-shaped cross-section, rectangular and other polygonal cross-sections, and of guides with gas or liquid surrounds instead of conventional solid cladding. It is also possible to use metal-covered fibers.
"Microbending" sensors are designed to take advantage of this loss mechanism. They generally involve a mechanical structure such as a serrated plate that presses on the fiber, producing a series of substantial local curvatures (bends). The loss of light is used as a signal to indicate displacement of the mechanical structure. Microbending sensors generally do not have a linear loss of light energy in response to changes in curvature, and are otherwise undesirable because of the necessity for a mechanical structure, and the strain which it imposes on the cladding and core of the fiber during deflection. If fibers are used without a mechanical structure to translate displacement into large local curvature, then the light loss due to bending is sufficiently large to be of practical use only when the bending is large. For small bends, such unenhanced microbending sensors are not useful because inadvertent bending of the fiber optic leads carrying light to and from the sensor portion of the fiber will produce changes in light loss that are indistinguishable from those produced by bends of the sensor portion. For these reasons, microbending sensors are generally not used in embedded applications, and rarely are used for measurements of curvature.
It is possible to treat optical fibers so that the amount of light travelling through the core changes more than usual with changes in curvature. Methods generally involve modification of the cladding so that it loses more light than usual over a short length. When straight, more light than usual is lost over the treated zone. When bent, additional amounts of light are lost due to the greater interaction of the treated sides with the light travelling through the fiber. Methods of treatment include abrasion, etching, heat treatment, embossing, and scraping of the cladding. Such treatment can produce a loss of light that is linear with curvature over a wide range, and which is much greater, by orders of magnitude, than the loss produced by microbending or by inadvertent bending of the leads carrying light to and from the sensitized zone.
A drawback of the above method of treatment is that loss is introduced even for a straight fiber, and the modification of the cladding can weaken glass fibers, especially if it involves removal of cladding around the entire circumference of the fiber.
It is undesirable to produce excessive light losses. If loss through the fiber is minimized, it is possible to use an inexpensive light emitting diode as a source of light, and to use inexpensive photodetectors and amplifiers to detect the amount of light being transmitted through the sensor. For this reason, it is desirable to make sensors with as little loss as possible when at the maximally transmissive end of their range (low residual light loss), but with as large a loss as possible due to a change in curvature (high sensitivity). Preferably, the loss should be a linear function of curvature, with the centre of the linear range being at the centre of range of the mechanical quantity (such as curvature or displacement) being measured. These requirements often cannot be met with known sensors, because parameters such as residual light loss and sensitivity cannot be varied independently. For instance, sensitivity to curvature increases as the length of the treated zone is increased, but so does residual light loss.
Harvill et al. (U.S. Pat. No. 5,097,252) have described intrinsic fiber optic sensors with the upper surface of the fiber treated to sense bending of fingers and other body parts. Although a monotonic output is claimed, the range of which includes a straight (zero curvature) sensor, the output is not linear and the range is not centred about zero curvature. Danisch (U.S. patent applications Ser. No. 07/738,560 filed on Jul. 31, 1991, and entitled Fiber Optic Bending and Positioning Sensor and Ser. No. 07/915,283 filed on Jul. 20, 1992, and having the same title, each naming Lee Danisch as the inventor, and further described in "Bend-enhanced Fiber Optic Sensors," SPIE: The International Society of Optical Engineering, L. A. Danisch, Volume 1795, 204-214, September, 1992, Boston, Mass., U.S.A.; "Smart Bone," Final Report for Canadian Space Agency Contract 9F006-1-0006/01-OSC, L. A. Danisch, 24 pp., June, 1992; and "Smart Wrist," Final Report for Canadian Space Agency Contract 9F006-2-0010/01-OSC, L. A. Danisch, March, 1993 has described fiber optic sensors with a surface of the fiber treated to emit light at a side with a minimal loss of throughput by means first described in U.S. Pat. No. 4,880,971, also in the name of Lee A. Danisch. The Danisch prior art includes descriptions of linear responses for a wide range of curvatures, a response that drops off as a cosine function for bends in planes not in the plane of maximum sensitivity, and a range centred about zero curvature. Another feature is a light absorbing coating which reduces or eliminates extraneous responses, including non-linearity. Control over the positioning of the centre of the range would open the possibility of mainly using the portion of the range with the highest light throughput (lowest loss), rather than that with the lowest throughput (highest loss) as taught in Harvill. This would be especially useful if the centre could be adjusted without affecting the residual light loss or sensitivity, or adversely affecting the strength of the fiber. The prior art does not teach how this can be done. The prior art describes sensors for which it is possible to vary the length, width and shape of a single treated strip, or the depth of multiple notches. Danisch, (U.S. Patent filings above) describes long sensors ". . . formed by alternating lengths of fibers with an emission strip with lengths of fully clad fibers." However, it is not shown how this technique can be used to gain control over the placement of the centre of the linear range.
A complicating factor in the manufacture of treated fiber optic sensors is that if their response to curvature is maximum in a given plane due to treatment not including the entire circumference of the fiber, then it can be difficult to maintain a proper orientation of the plane of maximum sensitivity after treatment but before embedment of the fiber. The main problem is the ease with which the fiber can twist about its long axis due to torques applied at any point along the length of the fiber. This is a problem for any fiber whose complete circumference is not treated, including fibers that are treated at both the top and bottom, thus having a response characteristic that does not distinguish upward from downward bends, but that distinguishes (through a cosine law) between up/down and left/right bends.
Another complicating factor in the design of many intensity-based fiber optic sensor system is the need for a return path and a means of reflecting or turning the light at the end of the fiber run.
To eliminate the need for a turnaround and return path, a coupler is often used at the measurement end, such that light can be injected into a single fiber with a reflector at the end. Injected light travels through the treated portion of the fiber, is reflected at the end, and returns to the coupler in the measurement system in the same fiber. The coupler is designed to extract the return light only, passing it on to a photodetector and amplifier. Unfortunately, the coupler introduces large losses and can be expensive to manufacture. Also, the reflective structure at the end can be lossy and difficult to manufacture.
In other cases, it is acceptable to use a return fiber with a reflective structure placed near the ends of the sensor and return fibers, whose distal ends face or are inserted into the reflective structure. Such a solution generally involves light leaving and re-entering the fibers, so that the sensor is no longer an intrinsic one, or it involves losses that may be unacceptable. It also invariably involves a reflective structure that is larger than the diameter of a single fiber or even two fibers, and is thus unacceptable for embedment.
A disadvantage of a turnaround loop at the distal end of a fiber optic sensing system is that even if sufficient width is available for the turnaround, it requires adding extra length to the system beyond the location of the sensor. This increases the size of the system and prevents sensing at the distal end of the system. For instance, it may be desirable to measure changes in curvature at the top end of a non-hinged but flexible lever which is being used as a "joystick" form of input device for a computer. If a turnaround loop is used for a fiber that enters the lever at the bottom, it would normally be at the top of the lever, thus not allowing known forms of curvature sensors to be placed at the top. As another example, if a turnaround loop is used and it is desired to measure curvature at the centre of a curved beam, then the beam must be long enough to accommodate the turnaround.
Another disadvantage of the turnaround is that it must be held in position to avoid changes in light intensity due to changes in curvature within the plane of the turnaround, particularly if the turnaround has a small radius of curvature which is producing light losses substantially greater than those of a straight fiber. If the turnaround is rigidly affixed to the substrate, this may produce stresses on the fibers between the turnaround and the location of the sensitized zone, which must also be rigidly attached to the substrate in order to properly sense its curvature.
However, if the disadvantages of the turnaround can be overcome, it has overwhelming advantages in terms of cost of manufacture, small size, lack of complexity, and relatively low light loss.
The present invention provides an improved sensor means for sensing curvature and displacement with minimum manufacturing cost and minimum damage to the fiber.
An object of the present invention is to provide a sensor means which minimizes residual light loss while optimizing sensitivity and preserving the strength of the fiber.
A further object of the invention is to provide a sensor means which allows maximum utilization of the portion of the linear range exhibiting the greatest transmission of light through the fiber.
A further object of the invention is to provide a sensor means that allows achieving a given residual light loss and sensitivity over a range of sensitized zone lengths.
A further object of the invention is to provide a sensor means which allows placing the sensitized zone of the sensor near the distal end of the sensor system.
A further object of the invention is to provide a sensor means which maintains the orientation of the sensitized zone, once treated.